THE Mexican diploid tuberous Solanum series Bulbocastana and Cardiophylla are important potential sources of genes for resistance to late blight of potatoes. Niederhauser and Mills1 noted field immunity in certain clones of S. bulbocastanum in their field trials in Mexico. No investigator has hitherto reported hybrids between these series and the Tuberosa. A knowledge of the crossability relationships of the Mexican diploid species may be useful in overcoming their apparent isolation.